Rheostat



(No Model.) 2 SheetsSheet 1.

E. WESTON.

RHEOSTAT. 110. 446,490. Patented Feb. 17,1891.

(No Model.) 2 Sheets-Sheet 2.

E. WESTON. I RHEOSTAT No. 446,490. Patented Feb. 17, 1891.

WITNESSES INVENTOR IMWM UNITED STATES PATENT OFFICE.

EDXVARD XVESTON, OF NEVARK, NEW JERSEY.

RH EOSTAT.

SPECIFICATION forming part of Letters Patent No. 446,490, dated February 17, 1891.

Application filed Fehruary28,1889. Serial No. 301,501. (No model.)

To all whom it may cancer/L.-

Be it known that I, EDWARD WESTON, of Newark, Essex county, New Jersey, have invented a new and useful Improvement in Rheostats or Potentiometers, of which the following is a specification.

My invention relates to an instrument having for its object the interposition of a desired electrical resistance in a circuit, the de termination of a resistancethe magnitude of which is unknown, the finding the difference of potential between two points in a circuit, and any other purposes to which it may be conveniently and usefully applied in the art of eleetro-measurcment.

My invention consists in a movable body of conducting material arranged and operating to traverse a continuous spiral, also of conducting material, the circuit-terminals being connected to said body and to one end of said spiral, by which means the length of the spiral interposed in the circuit, either main or shunt, may be regulated with great accuracy and facility, and also in the construction and ar 'angement of the instrument, substantially as hereinafter specified and claimed.

In the accompanying drawings, Figure 1 is a plan view of my new rheostat or potentiometer. Fig. 2 is a vertical section on the line X X of Fig. 1, and Fig. 3 is a side elevation.

Similar letters and figures of reference indicate like parts.

A is a cylindrical d ru m, of wood or other nonconducting material, supported on a standard.

D, of like material, provided with feet 0.

Upon the periphery of the drum A, and preferably countersunk therein, is spirally wound a metallic conductor D, of uniform thickness and homogeneous material, oiferin g consequently a uniform resistance at all points to an electrical current passing through it.

Passing through the center of the drum A is a sleeve E, having a flange F resting upon the surface of said drum and secured thereto by screws, as G. The sleeve E forms a hearing or nut for the cylindrical shaft H, which passes through it. On the upper end of said shaft is a threaded projection l, which passes through the radial arm J and receives the nut K, whereby said arm J is rigidly secured to the upper end of the shaft H. The arm J extends radially above the upper surface of the drum A, and then turns downward to extend across its periphery. Said arm J is provided with a fixed handle B, so that the arm J may be operated as a crank-arm to rotate the shaft II in the sleeve E. On the shaft H is cut a screw-thread at L, and on the inner periphery of the sleeveE is provided a fixed pin M, Fig. 2, and dotted lines, Fig. 1, which enters the groove of said thread. Consequently when the shaft ll is turned it is simultaneously caused to move longitudinally upward or downward in accordance with the direction of its rotation. The pitch of the screw L equals the pitch of the spiral formed by the conductor D.

At the lower extremity of the bent end of the arm J is provided a metallic brush or wiper N, which always bears against the exposed outer surface of the conductor 1), and hence, by reason of the like pitch of said conductor and the screw L, follows said conductor.

Upon the downwardly-hent portion of the arm J is secured an index-scale 0, having its edge parallel to the axial line of the drum A and marked in as many divisions as there are spaces between the turns of the spiral conductor D, each division being equal to the interval between successive turns of said conductor.

Upon the surface of the drum A is secured a circumferential scale P, laid off in equal divisions, here marked as one thousand.

Upon the periphery of the standard B is a binding-screw 1, which connects by a wire a, dotted lines, Fig. 3, with the upper end of the conductor D. A second binding-screw 2, also upon said standard, connects by a wire I) with the lower end of said conductor. A third bindingscrew 3, also upon said standard, connects by a wire 0 to the lower end of the shaft H.

I may use this instrument either as a rheostat or resistance-coil or as a potentiometer. As a rheostat it is operated in the following manner: One circuit-terminal is connected to binding-post 3 and theothercircuit-terminal to binding-post 1. The resistance of the whole length of conductor D is known, and hence, said conductor being of, homogeneous mate rial and of uniform thickness, it follows that the resistance of any fractional portion of said IOG conductor is proportional to the-length of said p0rtion.- Consequently, in order to intcrpose in the circuit a desired resistance less than that of the whole conductorD, it is necessary simply to interpose alength of said conductor bearing to the total length a ratio equal to that of the desired resistance to the whole resistance. Assume the total length of the conductor D to be six meters and the drum A to measure circumferentially one meter. Hence when the arm J has been carried around one complete revolution the brush N ontheend thereof will have passed over one-sixth of the total length of conductor D, or one meter. If the arm J is carried around but half a revolution,- thenythebrush N will have traversed bu -t one-half meter of the cond u'ctorD, and sofor; any fractional part. The scale P' being divided into thousandth sth erefore shows millimeters, and from this scale the length of the-arc' swept; over by the arm J, and hence the length of-thc" conduct-or passed over, may be read.

the drawings the brush- N is shown in contact with the" lower end of the conductor D. Hence tlfiewhole'lengthof said conductor is interposed in theci rc'uit between the hinding-screws 1 and 3, be turned around one complete revolution. The shaft H then moving upward a distance equal-to that between thelast and nextto the last turn of the conductor, the brush N will touch the next to the last turn at a point directly above that at which it before touched the last tu'rn.- Hence the last turn of the conductor will be cut out of circuit;v or, in other words, but five-sixths of the whole length, or five meters, will be included in the circuit. Another complete revolution of the brush will leave but four meters of the conductor incircuit, another tu-rn three meters,- and so on. It will be seen from the drawings that when the whole conductor D is in circuit the zero-mark on the scale 0 comes opposite the circumferential edge of the scale" I. Inasmuch asthedivisi'ons of the scale-O are each equal to the interval between two turnsof thecoiled conductor D, it follows that when the armhas made one complete revolution t-hcdivisionmarked1 ofthescale Q will come opposite theedge of scale I, and thus as each successive revolution is made the scale 0', being carried upward, will indicate the number of complete revolutions by thenumber which approximates the edge ofscale P. Suppose now, merely for illustration, that the total resistance of the conductor Dis six hundred ohms. Then the resistance of each complete turn of said conductor is one hundred ohms. Assume that it be desired to" intcrpose in the circuit aresistance of 275.5 ohms. Starting from the posit-ion shown in the drawings, the arm J is carried around until the number "2 on-the scale 0 comes opposite; the edge of scale P. The operator then continues to move the arm J until its edge reaches the 750 mark on the scale Suppose thearm J to P. He will then have two hundred and seventy-five ohms resistance in circuit. 7 .Then, as each division of the scale P represents onefifth of an ohm, he continues moving the arm forward over two and a half of these divisions, when the total desired resistance above noted will be interposed, or, conversely, supposing it be desired to measure an unknown resistance, as with a Wheatstone bridge,when a balance, for example, is secured when the scale 0 shows the number 3 just above the edge of scale P, and onthe scale? the edge of the arm J reaches the division marked2l0. The resis'tance'in' ci'rcuitwill then be three hundred and twenty-one ohms. Of course the tomuch' smaller than that here chosen for illustrative purposes-as,- for example, but SIX ohm, in which the measurement will be to one siX-thousandthof an ohm, and so on, as

laid off in smaller divisions, and thus still greater accuracy of measu-re'me'nt attaine'd. It will bea'pparent that in any event the manipuinthe same way. The operator has simply to rotate the arm J until the desired scalereading is obtained, or until in resistance measurement the balance is found.

' I will now describe the use-of the instrument in the measurement of thedifiere'nceof potential between two points on a circuit. It

of a battery, for example, whereit is-hi-ghest, down to the minus pole,- where it islowest. The fall of potential between any two points is proportional to the resistance between those points, so that if we go around a circuit to' a point where the potential has fallen through half its value then the current has at that point gone through half the res-istthe resistance the potential will have fallen to half its value. Now if the part of the circuit on which we desire to measure the potential difference consists, as in the present instrument, of a conductor of uniform thickness and offers a uniform resistance to the current, the potential will fall uniformly, sothat wecan measure dilferences in potential with great accuracy by simply interposing an ing the current between the terminals of a shunt in which a voltmeter isi'ncluded.

In Fig. 1 let one terminal of the main circuit, as (1, connect with the binding post 2 and the other terminal 6 connect with bindiug-post 1, the current passing through the whole conductor D in the direction of the arrows. Let a voltmeter f be connected in the shunt g with posts- 2 and 3. From what has been a-lready explained it will be apparent thatby moving the arm J any fraction of the ohms, inwhich case'the instrument will H1638 ure tothousand'th-s of an ohm, or but one desired; and equally the scale P may be lationand reading of the instrument are done ances; or, conversely, if we go through half tal resistance of the conductor may be very iswell known that the potential decreases all theway round a circuit from the plus pole IIO exactly-known len gth of theconductor carrywholc'length of the conductor D can be thrown into the shunt g and this length accurately adjusted from the scale-readings. The voltmeter will then show the difference in potential existing between the slmnt-terminals.

I claim 1. An electrical incasuring-instrument contaiuing a fixed cylindrical support of non-conducting material, a fixed conductor disposed spirally upon the periphery of said cylinder, a conducting-bod y, and means for moving said conducting-body along the length of and in contact with said conductor, substantially as described.

2. An electrical measuring-instrument containing a fixed spiral conductor, a conducting-body, and means for moving said body along the length of and in contact with said conductor, and circuit-connections com municating, respectively, with the ends of said spiral conductor and with said movable body, substantially as described.

3. An electrical measuring-instrument coniainin g a spiral conductor, a movable conducting-body in contact with and traversing said conductor, and a fixed indexscale disposed concentrically with said spiral conductor in proximity to said movable body and showing the extent of movement of said body in arc, substantially as described.

at. An electrical 1neasuringinstrument containing a spiral conductor, a movable conducting-body in contact with and traversing said conductor, and an index-scale disposed transversely, the turns of said conductor moving coincidently with said movable bodyan d showing the extent of movement of said movable body along said spiral, substantially as described.

5. An electrical ineasuring-instrument containin g a cylindrical support, a conductor spirally disposed upon the periphery thereof, an arm disposed in proximity to said periphery parallel to the axis of said cylinder and movable both in the direction of said axis and in a circular path around said periphery, and a body of conducting material supported by said arm in contact with said spiral conductor, substantially as described.

6. An electrical measuring-instrument containing a cylindrical support, a conductor spirally disposed upon the periphery thereof, an arm disposed in proximity to said periphery parallel to the axis of said cylinder and movable both in the direction of said axis and in a circular path around said periphery, a body of conducting material supported by said arm in contact with said spiral conductor, and a scale upon said arm marked in divisions respectively proportional to the-pitch of said spiral conductor and having its edge parallel to the axis of said support, substantially as described.

7. An electrical ineasuring-instrument containing a cylindrical support, aconductor spirally disposed upon the periphery thereof, a rotary shaft or rod havingits axis coincident with the axis of said cylinder, the said shaft being movablelongitudinallyin the direction of its axis, an arm extending radially from said shaft over one of the circular faces of said cylinder and downward in proximity to the periphery of said cylinder, a conductingbody supported by said arm in contact with said spiral conductor, and an index-scale upon said cylinder-face, substantially as described.

8. An electrical measuringdnstrumcnt containing a cylindrical support, a conductor spirally disposed upon the periphery thereof, a rotary shaft, a rod having its axis coincident with the axis of said cylinder, the said shaft being thread ed and turning in a fixed nut, an arm extending from said shaft, andaconducting-body supported by said arm in contact with said spiral conductor, the pitch of the thread upon said shaft being equal to the pitch of the spiral conductor, substantially as described.

O. The combination of the drum or cylinder A, the spiral conductor D on the periphery thereof, threaded shaft H, nut E, concentrically disposed in and with said cylinder, arm J upon said shaft H, brush N, and circuit-connections, substantially as described.

EDWARD WES OX.

Witnesses:

PARK BENJAMIN, M. Boson. 

